1. Field of the Invention
The invention relates to a switchable bandpass filter for a multiband tuner, comprising at least a capacitance diode for frequency-tuning of at least a resonant circuit of the bandpass filter, and a switching diode arranged at the input and the output, said switching diode being switchable to either the blocked state or to the conducting state by means of DC voltages.
2. Description of the Related Art
A tuner circuit including two parallel-arranged switchable bandpass filters is described in German Patent Specification 39 39 741. In this circuit, the switching is performed by means of switching diodes arranged at the input and the output of the bandpass filters. Furthermore, capacitance diodes for tuning the bandpass filters are provided within these bandpass filters. It is true that the switching diodes can improve the mutual decoupling of the bandpass filters, but the blocked bandpass filter also has a relatively low passband attenuation so that not only the signal of the bandpass filter which is switched on appears at the junction point of the two bandpass filters, but also the signal of the bandpass filter which is apparently switched off appears to an insufficiently attenuated extent. Thus, the frequency range processed by the blocked bandpass filter is not sufficiently attenuated.